Cationically photocurable epoxy resin compositions

ABSTRACT

The present invention provides a cationically photocurable epoxy resin composition, which contains (a) an epoxy resin component comprising an alicyclic epoxy resin and an aromatic-ring containing epoxy resin, (b) a cationic photoinitiator component, and (c) a filler selected from oxides, hydroxides and carbonates containing a Group II element. This composition exhibits improved adhesive strength to a glass such as an alkali glass, particularly a sodium-containing glass, or a metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a photocurable resin, in particular, to anepoxy resin composition curable by cation polymerization induced byphotoirradiation.

2. Description of the Related Technology

A cationically photocurable epoxy resin composition which can be curedby cationic polymerization of an epoxy resin induced by photoirradiation, in particular irradiation with light within the UV range,has many advantages; for example, it provides cured products havingphysical properties comparable with those from a thermosettingcomposition, it can be less inhibited by oxygene than a radical UVcurable composition, and it exhibits a smaller shrinkage than a radicalUV curable composition. It has been, therefore, used in variouswell-known applications such as a liquid crystal display and laminationin a digital video disk.

The resin composition comprises a cationic photoinitiator. It is knownthat a cationic photoinitiator is usually represented by a salt form,i.e., A⁺B⁻, and that a curing rate for the resin compositionsubstantially depends on the type of the anion B⁻. For example, it isknown that curing is quicker for a resin composition comprising an SbF₆⁻ containing compound such as triarylsulfonium hexafluoroantimonate or aB(aryl)₄ ⁻ containing compound such astetrakis(pentafluorophenyl)borate-{methyl-4-phenyl(methyl-1-ethyl)-4-phenyl}iodonium salt as a cationic photoinitiator.

However, our studies have revealed that while an epoxy resin compositioncomprising such a quickly-curing initiator exhibits good adhesivestrength to a non-alkali glass such as a borosilicate glass, it exhibitsinsufficient adhesive strength to an alkali glass such as a soda-limeglass or a metal such as stainless steel.

On the other hand, it is known that curing is slower for a resincomposition comprising a PF₆ ⁻ containing compound such as atriarylsulfonium hexafluorophosphate as a cationic photoinitiator. Ourstudies have revealed that when used in combination with aquickly-curing initiator, such a slowly-curing cationic photoinitiatorimproves adhesive strength in adhesion to an alkali glass or a metal tosome extent, but not adequately. In some cases, a quickly-curinginitiator must be incorporated in order to give a required curing ratefor a particular application. It has been, therefore, strongly needed toimprove adhesive strength.

Japanese Patent Publication JP-A 6-73159 has described that talc orcordierite is added to a cationically photocurable epoxy resin. The JP'159 publication described that it is essential to use a fluorinatedepoxy resin and has listed cordierite powder, talc powder and silicapowder as a filler added to a fluorinated bisphenol-A type of epoxyresin. Furthermore, only adhesiveness to BK-7 glass, which does notcontain an alkaline metal, has been studied in a test for adhesion to aglass. Thus, the JP '159 publication has disclosed no cationicallyphotocurable resins which provide adhesive strength adequate foradhesion to an alkali glass or to a metal such as stainless steel.

SUMMARY OF THE INVENTION

An objective of this invention is thus to provide a cationicallyphotocurable epoxy resin composition which can be used as an adhesiveexhibiting improved adhesive strength to a glass such as an alkaliglass, particularly a sodium-containing glass, or a metal; inparticular, to provide a cationically photocurable epoxy resincomposition which can be used as an adhesive having an adequate curingrate while exhibiting higher adhesive strength to an alkali glass or ametal, even when using a salt represented by A⁺B⁻ where the anion B⁻ isSbF₆ ⁻ or B(aryl)₄ ⁻ as a cationic photoinitiator.

This invention provides a cationically photocurable epoxy resincomposition comprising (a) an epoxy resin component comprising analicyclic epoxy resin and an aromatic-ring containing epoxy resin, (b) acationic photoinitiator component, and (c) a filler selected from thegroup consisting of oxides, hydroxides and carbonates containing a GroupII element in the long periodic table.

In this invention, addition of an inorganic compound containing a GroupII element, especially in the form of oxide, hydroxide or carbonate,unexpectedly significantly improve adhesive strength to an alkali glassor metal. A cationically photocurable epoxy resin of this invention isthus very useful as an adhesive when at least one of two materials to beattached is an alkali glass or metal.

Although a mechanism for improvement of adhesive strength is unknown, webelieve that a material suitable to a filler may have appropriatealkalinity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate methods for determining a glass/glass tensileadhesive strength and a stainless steel/alkali glass tensile adhesivestrength, respectively, where 1 and 2 represent a glass; 3 represents asoda-lime glass; 4 represents a stainless steel pin; and 5 representsresin composition (adhesive).

DETAILED DESCRIPTION OF THE INVENTION

A cationically photocurable resin composition of this inventioncomprises at least (a) an epoxy resin component comprising an alicyclicepoxy resin and an aromatic-ring containing epoxy resin, (b) a cationicphotoinitiator component, and (c) a filler selected from the groupconsisting of oxides, hydroxides and carbonates containing a Group IIelement in the long periodic table, as described above.

The epoxy resin component comprises an alicyclic epoxy resin and anaromatic-ring containing epoxy resin. Although a fluorinated epoxy resinmay not be necessarily excluded from epoxy resins used in the epoxyresin component, there is no need to incorporate it in the presentcomposition. In one embodiment of this invention, a non-fluorinatedepoxy resin is used as a main ingredient, i.e., at least 60% by weight,preferably at least 80% by weight of the epoxy resin component. Inparticular, it is also preferable to use at least 60% by weight,particularly at least 80% by weight of a non-fluorinated resin in anaromatic-ring containing epoxy resin. In the most preferable embodiment,all the epoxy resins are non-fluorinated resins.

An alicyclic epoxy resin has an epoxy group associated with a structurehaving ring distortion such as a cyclohexene oxide and a cyclopenteneoxide structures in molecule, preferably a structure having two or moreof this type of epoxy group in one molecule. Typical examples of thealicyclic epoxy resin include those represented by formulas (1) to (5).

Examples of the aromatic-ring containing epoxy resin include bisphenolepoxy resins such as bisphenol-A epoxy resins, bisphenol-F epoxy resinsand bisphenol-S epoxy resins; novolac epoxy resins such as phenolnovolac epoxy resins and cresol novolac epoxy resins; and biphenyl epoxyresins such as YX4000 (trade mark; Japan Epoxy Resin Co. Ltd.). Thearomatic-ring containing epoxy resin usually has at least one epoxygroup in one molecule. The epoxy equivalent of the resin may be selectedas appropriate.

The epoxy resin component may comprise other known diluents which may beepoxy-containing resins, vinyl ethers, oxetane compounds and polyols.Specific examples include aliphatic alkyl mono- or di-glycidyl etherssuch as cyclohexanedimethanol diglycidyl ether, butyl glycidyl ether,2-ethylhexyl glycidyl ether and allyl glycidyl ether; alkyl glycidylesters such as glycidyl methacrylate and tertiary carboxylic acidglycidyl esters; styrene oxide; aromatic alkyl monoglycidyl ethers suchas phenyl glycidyl ether, cresyl glycidyl ether, p-s-butylphenylglycidyl ether and nonylphenyl glycidyl ether; tetrahydrofurfurylalcohol glycidyl ether; mono- or poly-functional vinyl ethers such ascyclohexanedimethanol divinyl ether, triethleneglycol divinyl ether andhydroxybutyl vinyl ether; polyols represented by general formulaH—(OCH₂CH₂)_(n)—OH where n is an integer of one or more, generally about2 to 20; and oxetane compounds represented by formula (6).

The content of the alicyclic epoxy resin is 1 to 80% by weight,preferably 5 to 40% by weight, more preferably 10 to 30% by weight tothe total amount of the epoxy resin component consisting of thealicyclic epoxy resin, the aromatic-ring containing epoxy resin and thediluent. The diluent is an optional ingredient and is added to a contentof 0 to 40% by weight, preferably 0 to 20% by weight to the total amountof the epoxy resin component.

A cationic photoinitiator is a salt represented by A⁺B⁻. The compositionaccording to this invention comprises at least an initiator in which theanion B⁻ is SbF₆ ⁻ or B(aryl)₄ ⁻ ion such as B(C₆F₅)₄ ⁻ (hereinafter,referred to as “the first initiator”). The B(aryl)₄ ⁻ ion may be, inaddition to B(C₆F₅)₄ ⁻, B(C₆F₄OCF₃)₄ ⁻ or B(C₆F₄CF₃)₄ ⁻. The cation A⁺is preferably an aromatic iodonium ion or aromatic sulfonium ion. Theexamples of the compounds include the compounds represented by thefollowing formulas.

An initiator in which the anion B⁻ is SbF₆ ⁻ or a B(aryl)₄ ⁻ ion, i.e.,the first initiator, provides a higher curing rate. However, in acomposition without a “filler as defined in this invention” describedlater, adhesiveness is satisfactory to a non-alkali glass while beingconsiderably reduced to an alkali glass or a metal. This invention is,therefore, quite effective for a composition containing the firstinitiator.

In addition to the first initiator, an additional initiator in which theanion B⁻ is an anion other than SbF₆ ⁻ or a B(aryl)₄ ⁻ ion (herein afterreferred as the “second initiator”) may be added. The second initiator,which is an optional ingredient, has an advantage of improving adhesivestrength, despite that it exhibits a low curing rate in comparison withthe first initiator. Therefore, it can be used in combination with thefirst initiator. The examples of the anion B⁻ in the second initiatorinclude PF₆ ⁻, AsF₆ ⁻ and BF₄ ⁻. Specific compounds include theabove-mentioned salts in which SbF₆ ⁻ is replaced with PF₆ ⁻, AsF₆ ⁻ orBF₄ ⁻.

The content of the second initiator may be determined in considerationof the application of the composition, adhesive strength required and acuring rate, and is generally selected such that a weight ratio of (thesecond initiator)/(the first initiator) is substantially within therange of 0/1 to 10/1.

A cationic photoinitiator is preferably added, for example, at 0.1 partsby weight to 10 parts by weight, preferably 0.5 parts by weight to 5parts by weight to 100 parts by weight of the epoxy resin component.

It is also preferable to add a photosensitizer such as thioxanthone asnecessary.

A filler used in this invention is preferably selected from the groupconsisting of oxides, hydroxides and carbonates containing a Group IIelement in the long periodic table. Group II elements in the longperiodic table belong to the former Group IIA. The preferable element ismagnesium, calcium or barium. They may take a form of complex oxide orcomplex hydroxide of a Group II element with another element such asaluminum and silicon (including those which can be expressed as acomplex oxide or complex hydroxide in composition formula). A filler asdefined herein is referred to as a “filler as defined in thisinvention”.

Examples of a magnesium-containing filler include MgO, Mg(OH)₂, talc(2MgO—SiO₂(OH): magnesium silicate hydroxide), cordierite (2MgO-2Al₂O₃),magnesium meta-silicate and magnesium ortho-silicate. Among these, talcand cordierite are particularly preferable.

Preferable examples of a calcium- or barium-containing filler includecarbonates such as calcium carbonate and barium carbonate.

A particle size of the filler as defined in this invention is generally0.5 to 15 μm, preferably 5 μm or less.

Even a small amount of the filler as defined in this invention mayimprove adhesive strength to a certain extent if is present in thecomposition. Its content may be, therefore, more than zero to 100 partsby weight of the epoxy resin component, preferably at least 1 part byweight or more, more preferably at least 5 parts by weight or more, mostpreferably at least 10 parts by weight or more. The upper limit may bedetermined in consideration of the properties such as handlingproperties and physical properties. It is generally 300 parts by weightor less, preferably 200 parts by weight or less, particularly 100 partsby weight or less.

In this invention, another filler such as silica and alumina may beadded if necessary. In general, the total amount of the filler asdefined in this invention and the additional filler is 300 parts byweight or less to 100 parts by weight of the epoxy resin component.

A composition of this invention may further comprise a silan couplingagent, a pigment, a surfactant, a preservative, a plasticizer, alubricant, a defoamer and a leveling agent.

Examples of a silane coupling agent include, but not limited to,γ-aminopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane,γ-methacryloxypropyltrimethoxysilane, γ-glycidoxypropyltrimethylsilane,SH6062 and SZ6030 which are available from Toray-Dow Corning SiliconeInc., and KBE903 and KBM803 which are available from Shin-Etsu SiliconeInc.

A cationically photocurable epoxy resin composition according to thisinvention may be suitably used as an adhesive for an application whereat least one of the materials to be bonded is an alkali glass or metal.Typical examples of an alkali glass include soda-lime glasses. Even whena transparent electrode such as ITO and tin oxide is formed on thesurface of the glass, improved adhesive strength may be provided.Examples of a metal to be attached include, but not limited to,stainless steel, iron and copper.

EXAMPLES

The ingredients shown in Table 1 were blended to prepare a resincomposition. In Example and Comparative Examples, a bisphenol-A epoxyresin was RE-3015 (Nippon Kayaku Co., Ltd.); an alicyclic epoxy resinwas bis(3,4-epoxycyclohexyl) adipate (Union Carbide Inc.; UVR-6128); anda silane coupling agent was γ-glycidoxypropyltrimethylsilane.

For a cationic photoinitiator in the table, the “rapid-curing initiator”was the compound represented by formula (C-9),tetrakis(pentafluorophenyl)borate-{methyl-4-phenyl(methyl-1-ethyl)-4-phenyl}iodoniumsalt; and the “slow-curing initiator” was a mixture of the compoundsrepresented by formulas (C-5) and (C-6), CYRACURE™ PhotoinitiatorUVI-6990 (Union Carbide Inc.).

The silica used has a specific gravity of 2.635 and an average particlesize of 1.5 μm.

The talk used has a specific gravity of 2.70 and an average particlesize of 5 to 6 μm.

Measurements were conducted as follows.

Tack-Free Time

On a glass plate was applied each resin composition to a thickness of 1mm and the product was irradiated with light from its surface. A lightsource used was a medium pressure mercury lamp (Eyegraphics Co.,HO15-L312), which was used in measurement described below and wasadjusted such that a light intensity at 365 nm was to be 100 mW/cm² asdetermined at 365 nm with a UV meter (Ushio UIT101). Then, anirradiation time until tack on the surface of the resin compositiondisappeared was determined as a “tack-free time”. A shorter value ispreferable for the time because it indicates that tack on the surfacemore quickly disappears.

Cured Through Volume

For determining a curing depth from the surface, each resin compositionwas placed in a cylindrical Teflon mold with a diameter of 1.5 cm and adepth of 1 cm, and was then irradiated at a light intensity of 100mW/cm² at 365 nm for 30 sec. The product was removed from the mold, thelower uncured composition was wiped off, and the height of the residualcured resin was determined as a “cured through volume” for comparison. Alarger height is more preferable because it indicates that curingproceeds deeper from the surface.

Adhesive Strength

Tensile adhesive strengths between a non-alkali glass and a non-alkaliglass and between an alkali glass and an alkali glass were determined bylaminating two glass plates 1, 2 with the dimensions of 1 mm(thickness)×15 mm (width)×30 mm (length) via a resin composition(adhesive) as a cross as shown in FIG. 1, irradiating the product withlight of intensity of 100 mW/cm² at 365 nm for 30 sec, post-heating itat 100° C. for 1 hour to cure the resin composition and then pullingthese glasses to the arrowed direction in FIG. 1 such that these areseparated from each other.

The non-alkali glass was Corning 1737 glass and the alkali glass was asoda-lime glass.

A tensile adhesive strength between a stainless steel pin and an alkaliglass was determined as shown in FIG. 2 by applying a resin composition(adhesive) 5 between a soda-lime glass 3 with the dimensions of 6 mm(thickness)×31 mm (width)×38 mm (length) and a stainless steel pin 4with the dimensions of 12.7 mm (diameter)×38 mm (length), curing it asdescribed above for the adhesion between glasses; and then pulling thestainless steel pin 4 to the arrowed direction while fixing thesoda-lime glass 3.

TABLE 1 Comp. Comp. Comp. Components Example 1 Example 1 Example 2Example 3 Bis A Epoxy 43.17 22.80 30.00 27.00 Cycloaliphatic 8.71 15.1923.38 11.34 epoxy Silane coupling 1.88 1.00 2.00 1.00 agent Rapidinitiator 0.19 1.00 0.60 0.15 Slow initiator 0.47 — 1.00 0.50Thioxantone 0.01 0.01 0.02 0.01 Silica 26.80 60.00 43.00 60.00 Talc18.77 — — — Total: 100.00 100.00 100.00 100.00 Tack-free time <5 s <5 s<5 s <5 s Cured through 1.4 mm >4.7 mm 1.9 mm >1.9 mm volume Tensile 2.6N/mm² NA NA NA Strength (Non-alkaline/ Non-Alkaline Glass) Tensile 1.7N/mm² 0.8 N/mm² 1.0 N/mm² NA Strength (Alkaline/ Alkaline Glass) Tensile7.8 N/mm² 5.7 N/mm² NA 5.5 N/mm² Strength (Steel Pin/ Glass) NA: notmeasured

As described above, this invention can provide a cationicallyphotocurable epoxy resin composition which can be used as an adhesiveexhibiting improved adhesive strength to a glass, particularly an alkaliglass, or a metal. In particular, it can be used as an adhesive havingan adequate curing rate while exhibiting higher adhesive strength to analkali glass or a metal, even when using a salt represented by A⁺B⁻where the anion B⁻ is SbF₆ ⁻ or B(aryl)₄ ⁻ as a cationic photoinitiator.

1. A cationically photocurable epoxy resin composition comprising: (a)an epoxy resin component comprising the combination of an alicyclicepoxy resin in an amount of about 1% to about 40% by weight of the epoxyresin component and non-fluorinated aromatic-ring containing epoxy resinin an amount of at least 60% by weight of the epoxy resin component; (b)a cationic photoinitiator component; (c) a filler having a particle sizein the range of 0.5 to 15 μm selected from the group consisting of talc,Mg(OH)₂ and mixtures thereof; and (d) silica.
 2. The composition asclaimed in claim 1 wherein (b) the cationic photoinitiator componentcomprises a salt represented by A⁺B⁻in which the anion B⁻ is SbF₆ ⁻ orB(aryl)₄ ⁻.
 3. The composition as claimed in claim 2 wherein (b) thecationic photoinitiator component further comprises an initiatorrepresented by A⁺B³¹ in which the anion B⁻ is other than SbF₆ ⁻ orB(aryl)₄ ⁻,in addition to the initiator represented by A⁺B⁻ in which theanion B⁻ is SbF₆ ^(− or B(aryl)) ₄ ⁻.
 4. The composition as claimed inclaim 3, wherein the initiator in which the anion B⁻ is other than SbF₆⁻ or B(aryl)₄ ⁻ is an initiator in which the anion B⁻ is selected fromthe group consisting of PF₆ ⁻, AsF₆ ⁻ and BF₄ ⁻.
 5. The composition asclaimed in claim 1, wherein the filler in (c) is talc.
 6. Thecomposition as claimed in claim 5, wherein the silica is present in anamount greater than an amount of talc.
 7. The composition as claimed inclaim 1, further comprising a silane coupling agent.
 8. The compositionas claimed in claim 1, wherein the aromatic-ring containing epoxy resincomprises at least one selected from the group consisting of bisphenolepoxy resins, novolac epoxy resins and biphenyl epoxy resins; and thealicyclic epoxy resin comprises at least one selected from the compoundshaving cyclohexene oxide or cyclopentene oxide structure in molecule. 9.The composition as claimed in claim 1, wherein the epoxy resin componentconsists essentially of non-fluorinated resins.
 10. The composition asclaimed in claim 1, disposed as an adhesive between two substrates, atleast one of which is an alkali glass or metal.
 11. The composition asclaimed in claim 1, disposed as an adhesive between two substrates, atleast one of which is an alkali glass.
 12. The composition as claimed inclaim 1, wherein the epoxy resin component further comprises a diluent.13. A cationically photocurable epoxy resin composition comprising: (a)an epoxy resin component comprising the combination of an alicyclicepoxy resin in an amount of about 1% to about 40% by weight of the epoxyresin component and an aromatic-ring containing epoxy resin in an amountof at least 60% by weight of the epoxy resin component; (b) a cationicphotoinitiator component comprising a first initiator represented byA⁺B⁻, in which the anion B⁻ is SbF₆ ⁻ or B(aryl)₄ ⁻ and a secondinitiator represented by A⁺B⁻ in which the anion B⁻ is other than SbF₆ ⁻or B(aryl)₄ ⁻; (c) a filler having a particle size in the range of 0.5to 15 μm selected from the group consisting of talc, Mg(OH)₂and mixturesthereof; and (d) silica.
 14. The composition as claimed in claim 13,wherein the epoxy resin component consists essentially ofnon-fluorinated resins.
 15. A cationically photocurable epoxy resincomposition comprising: (a) an epoxy resin component comprising thecombination of an alicyclic epoxy resin in an amount of about 1% toabout 40% by weight of the epoxy resin component and an aromatic-ringcontaining epoxy resin in an amount of at least 60% by weight of theepoxy resin component; (b) a cationic photoinitiator componentcomprising a first initiator represented by A⁺B⁻ in which the anion B⁻is SbF₆ ⁻ or B(aryl)₄ ⁻ and a second initiator represented by A⁺B⁻ inwhich the anion B⁻ is selected from the group consisting of PF₆ ⁻, AsF₆⁻ and BF₄ ⁻; (c) a filler having a particle size in the range of 0.5 to15 μm selected from the group consisting of talc, Mg(OH)₂ and mixturesthereof; and (d) silica.
 16. The composition as claimed in claim 15,wherein the epoxy resin component consists essentially ofnon-fluorinated resins.